Tentative Detection of CO+ toward Cygnus A

Details Tentative Detection of CO+ toward Cygnus A Tentative Detection of CO+ toward Cygnus A

Dec 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000apj...545l.113f&link_type=abstract

The Astrophysical Journal, Volume 545, Issue 2, pp. L113-L116.

Astronomy and Astrophysics

Astronomy

14

Galaxies: Active, Galaxies: Individual: Name: Cygnus A, Galaxies: Nuclei, Ism: Molecules, Molecular Processes, Radio Lines: Galaxies

Scientific paper

We report the tentative detection of the two spin-rotation components of the CO+ N=1-->0 transition in absorption toward the nucleus of Cygnus A. This is the first molecular detection at millimeter wavelengths in Cyg A and the first detection of CO+ in an extragalactic source. In addition, a new upper limit to the CO absorption has been obtained, which improves upon previous limits by more than a factor of 40. Estimates of the CO+ fractional abundance based on a single rotational transition strongly depend on the excitation conditions. In fact, if the excitation is radiative, the estimated CO+ abundance changes from ~10-5 to 10-10 when the distance to the continuum source changes from 30 to 1000 pc. Based on models of the ionization, chemistry, and excitation of the molecular gas located within a few hundred parsecs of a luminous X-ray source like the nucleus of Cyg A, we obtain a reasonable agreement with the observations if the observed CO+ is concentrated at a distance of 350 pc from the X-ray source. The derived CO+ fractional abundance for this distance is 5×10-9, a factor of 50 larger than in the photodissociation regions of our Galaxy. Even though CO is not detected, its abundance could still approach ~10-4, in harmony with model predictions. The excitation of CO+ may be sensitive to its formation and destruction as well as to inelastic collisions and to radiative coupling with the millimeter-wave continuum source in the nucleus, thus making it a valuable probe of the circumnuclear environment of an active galactic nucleus.

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